Enhanced aerobic bioremediation of an aquifer heavily contaminated with a mixture of chlorobenzenes and hexachlorocyclohexanes at the Sardas landfill (Spain)

The groundwater at the Sardas landfill in Huesca, Spain, is contaminated with benzene, chlorobenzenes, and hexachlorocyclohexane (HCH) isomers due to illegal waste dumping from a former lindane factory. In this study, microcosms using field-derived groundwater to evaluate in situ bioremediation were constructed. Anaerobic biostimulation with lactate successfully transformed alpha-, beta-, delta-, and gamma-HCH within two weeks, but failed to degrade benzene and less chlorinated benzenes, even with nutrient addition. In contrast, aerobic biostimulation led to rapid degradation of benzene, chlorobenzenes, and alpha-, delta-, and gamma-HCH. Notably, adding a phosphorus source significantly increased the degradation rates. Following these laboratory results, an in situ pilot test using the oxygen-releasing compound CaO2 was conducted at two site injection wells. The field results mirrored those from the microcosms, showing a marked reduction in contaminants at both the injection wells and surrounding wells. Bacterial community analysis based on the 16S rRNA genes in samples derived from aerobic microcosms and groundwater before and after the biostimulation test revealed a marked increase in the genus Pseudomonas, suggesting its potential role as biodegrading agent. This study illustrates the effectiveness of biostimulation as a viable strategy for treating groundwater contaminated with HCH isomers, benzene, and chlorobenzenes.